Double stranded nucleic acid molecules (i.e., DNA, DNA/RNA and RNA/RNA) associate in a double helical configuration. This double helix structure is stabilized by hydrogen bonding between bases on opposite strands when bases are paired in one particular way (A+T/U or G+C) and hydrophobic bonding among the stacked bases. Complementary base paring (hybridization) is central to all processes involving nucleic acid.
In a basic example of hybridization, nucleic acid probes or primers are designed to bind, or “hybridize,” with a target nucleic acid, for example, DNA or RNA in a sample. One type of hybridization application, in situ hybridization (ISH), includes hybridization to a target in a specimen wherein the specimen may be in vivo, in vitro, in situ, or for example, fixed or adhered to a glass slide. The probes may be labeled to make identification of the probe-target hybrid possible by use of a fluorescence or bright field microscope/scanner. Such labeled probes can be used, for example, to detect genetic abnormalities in a target sequence, providing valuable information about, e.g., prenatal disorders, cancer, and other genetic or infectious diseases.
The efficiency and accuracy of nucleic acid hybridization assays mostly depend on at least one of three major factors: a) denaturation (i.e., separation of, e.g., two nucleic acid strands) conditions, b) renaturation (i.e., re-annealing of, e.g., two nucleic acid strands) conditions, and c) post-hybridization washing conditions.
In order for the probes or primers to bind to the target nucleic acid in the sample, complementary strands of nucleic acid may be separated. This strand separation step, termed “denaturation,” typically requires aggressive conditions to disrupt the hydrogen and hydrophobic bonds in the double helix. Once the complementary strands of nucleic acid have been separated, a “renaturation” or “reannealing” step allows the primers or probes to bind to the target nucleic acid in the sample. This step is also sometimes referred to as the “hybridization” step.
Traditional hybridization experiments, such as ISH assays, use high temperatures (e.g., 95° C. to 100° C.) and/or high concentration formamide-containing solutions (e.g., greater than 40%) to denature doubled stranded nucleic acid. However, these methods have significant drawbacks.
For example, heat can be destructive to the structure of the nucleic acid itself because the phosphodiester bonds may be broken at high temperatures, leading to a collection of broken single stranded nucleic acids. In addition, heat can lead to complications when small volumes are used, since evaporation of aqueous buffers is difficult to control.
Formamide is a solvent that has a destabilizing effect on the helical state of, for example, DNA, RNA, and analogs by displacing loosely and uniformly bound hydrate molecules and by causing “formamidation” of the Watson-Crick binding sites. Thus, formamide has a destabilizing effect on double stranded nucleic acids and analogs, allowing denaturation to occur at lower temperatures. However, although formamide lowers the melting temperature (Tm) of double-stranded nucleic acid, when used at high concentrations, it also significantly prolongs the renaturation time, as compared to aqueous denaturation solutions without formamide.
In addition, using formamide has disadvantages beyond a long processing time. Formamide is a toxic, hazardous material, subject to strict regulations for use and waste. Furthermore, the use of of formamide appears to cause morphological destruction of cellular, nuclear, and/or chromosomal structure.
Moreover, the use of formamide, while accepted as the standard technique for hybridization, is hampered by the long time required to complete the hybridization, depending on the conditions and the nucleic acid fragments or sequences used. For example, the denaturation step is followed by a longer time-consuming hybridization step, which, e.g., in a traditional fluorescent in situ hybridization (FISH) protocol takes 14-24 hours, and can even take up to 72 hours. Examples of traditional hybridization times are shown in FIGS. 1 and 2.
The step of re-annealing (i.e., hybridizing) two complementary strands of nucleic acid chains is by far the most time-consuming aspect of an assay using hybridization. Until now it was believed that the use of chaotropic agents, such as formamide, guanidinium hydrogen, and urea, which interfere with the Watson-Crick binding sites of nucleic acid bases and thereby disturb the hydrogen bonds between complementary nucleic acid bases, was one way to lower the melting temperature (Tm) of the complementary chains. However, although the use of chaotropic agents lowers the Tm, these agents appear to significantly prolong the hybridization time compared to hybridization in an aqueous solution without a chaotropic agent. Furthermore, besides the disadvantage of the long processing time, the use of formamide appears to incur morphological destruction of cellular, nuclear, and/or chromosomal structure. Finally, formamide is considered a toxic and hazardous chemical to humans.
In some embodiments, the present invention provides several potential advantages over prior art hybridization applications, such as faster hybridization times, lower hybridization temperatures, and less toxic hybridization solvents.